Field of the Invention
The invention relates to a method and screw for the mounting of boards/planks of fiber cement, plastics, composites, and/or other materials with compact and high density, and in general of hard nature, resulting in corresponding brittle characteristics, where internal stresses can cause breakage of the material, especially at imbricated mounting of boards/planks—irrespective of the underlying material on which mounting is performed, it being wood, metal, or other material. The invention particularly relates to a screw and use thereof, the screw having a first thread near the tip of the screw and a second, larger thread near the head and a milling section between the threads for adapting a screw hole to the diameter of the second thread.
Description of Related Art
When boards/planks of fiber cement, among others, are mounted in imbricated configuration, that is, partially overlapping as fish scales, it is customary to use either a nail with a thin, flat head or a countersunk screw, the latter often with different kinds of countersinking fins under the head, such that the screw can also be countersunk in the boards/planks and therewith does not protrude and inadvertently raise the board/plank mounted on top of the fastening.
Nails as well as traditional countersunk screws (with or without fins) exert a pressure of the board/plank against the underlying surface, which is unfortunate because it implies the risk of breakage when mounted in imbricated configuration (imbrication angle), or at least implies stress of the mounted board/plank due to the cavity underneath the board/plank (see FIG. 1 under “improper installation” and FIG. 2). In addition, the tensile stress created by the nail when penetrating the fiber cement plank implies a high risk for cracks in the plank.
Particularly, when using traditional countersunk screws (with or without fins), there occurs, in addition, often, risk of cleavage of the board/planks because the mount is often relatively close to the edge. The cause is pressure of the pre-stressing of the screw toward the underlying surface as well as internal pressure as a result of the penetration of the screw into the material (see FIG. 3).
Although, the screw or nail presses material radially aside from the penetration hole by pressure in the radial direction, the corresponding expansion of the material leads to a tangentially directed tensile stress around the hole, which implies a high risk for cracks in the fiber cement plank or leading to breakage.
Furthermore, nails also have several other undesirable characteristics, including the fact that, during assembly, a hammer is beaten towards a relatively fragile board/plank, where only a single failing hit can cause great damage; additionally, nails cannot be pulled out if they enter little too far (see FIG. 4 of “defective installation”).
In general, the variety of screws is vast, and different types of screws are specifically produced for certain uses. Often variations of screws appear small at first sight, however, the importance of special screw designs may be very large despite such very small differences among screws. Due to this very specialized design of screws targeting very specific uses, it is customary to use a screw that is dedicated for one specific type of use only in this specific field and not in neighboring specialized field. Screws for specific uses imply the following examples.
One type of examples is distance screws. Distance screws are built up by similar principle of two threads of different diameter but equal pitch, a milling section between the threads, and a head which is entering the panel, either because it is as narrow as the screw or due to fins that countersink the head.
German utility model DE 296 11 749 discloses a distance screw which has two different threads, a narrower one for the wall and a wider one for the panel or frame that is to be fastened to the wall. In order to keep the distance constant between the wall and the panel or frame, the threads need to have the same pitch. The screw as disclosed in this utility model has a milling section between the threads in order to adjust the hole diameter in the panel or frame to a precise value. In order for the screw to enter the frame or panel without pulling the frame or panel towards the wall, the screw has no head with a diameter larger than the screw diameter. The latter is also an important feature. A similar configuration for a distance screw is disclosed in German patent publication DE 44 01 908, although it comprises milling wings, whereas the German utility model DE 296 11 749 discloses milling ribs. In contrast to the German utility model DE 296 11 749, where no plugs are used in the wall for the screw, because the screw is configured to cut its way into concrete, British patent specification GB 1575194 discloses such screw with a plug in the wall, which is a traditional way for mounting frames. The screw in GB 1575194 is used for panels, for example of wood, wood shavings, plastics, or asbestos cement. These screws without a head that has a larger diameter than the stem of the thread are not suitable for quick mounting of thin fiber cement planks, as there is a risk that the screw is screwed all the way through the plank/board without stopping the mounting at the correct instance. Also, the lack of proper head implies the risk of the plank not being held properly against the underlying material. Therefore, for fiber cement planks/boards, screws are traditionally provided with relatively large heads.
In German patent publication DE 10 2004 018 069, a distance screw is disclosed with two threads and milling section between the threads, as well as a head that is wider than the screw. The wider head is provided in order to have a more stable holding of the panel against the wall. In order not to displace the panel during screwing, the head has milling fins underneath the head, which assist in countersinking the head into the panel. Alternatively, in DE 10 2004 018 069, the head has a hollow underside, but without milling fins. As the underside of the head of this screw is conical, such a screw is also not suited for fiber cement planks/boards, because the conical underside would induce radial pressure and, as explained above, tangential tensile stress, which could lead to cracks and breakage.
U.S. Pat. No. 3,682,507 by Waud discloses a different type of screw, specifically for insulation panels that have a soft foam and a thin sheet of metal on that side of the insulation panel that is remote from the wall. The screw has a drill tip for drilling through the metal sheet and the wall during mounting of the insulation panel against a wall. The screw comprises three sections of threads. During mounting procedure, the drill tip drills a hole in the outer metal sheet of the insulation material, the first thread is penetrating the insulation material through the drilled hole in the metal sheet, after which milling wings enlarge the hole in the metal sheet and a second thread engages with the rim of the hole in order to control the penetration speed of the screw through the soft insulation panel. This controlled speed is useful in order not to damage the drill tip when impinging on the wall. Once, the drill tip drills into the wall and the first thread engages with the wall, the third thread just below the head of the screw engages with the metal sheet. During the mounting, the metal sheet may have been slightly deformed in a local area around the hole due to press against it with the drill tip. In order not to maintain compressed insulation material below the hole, it is necessary to return the local area to the correct distance from the wall; for this reason, the third thread underneath the screw head has a pitch which is higher than the pitch of the first or second thread section, by which the metal sheet, but not the insulation material, is pulled slightly away from the wall and the remaining insulation material and towards the relatively large washer of the head.
As it appears, especially, from the example of U.S. Pat. No. 3,682,507, screws may be configured with very specific features and with dimensions specifically targeting the selected use thereof. For example, the screw of U.S. Pat. No. 3,682,507 would not be useful as a distance screw for overlapping fiber cement panels, such a planks/boards. First of all, the relatively bulky screw head would create a large distance between overlaying panels and prevent mounting of panels close onto each other. However, even with such head countersunk into the panel, the screw would still not make a close overlapping of panels possible, because the screw would not keep the distance between the panel and the wall because the higher pitch at the head region would pull the panels away from the wall in the final mounting step when the lower pitch thread is entering the panel. In this connection, it is pointed out the varying pitch in the screw of U.S. Pat. No. 3,682,507 is not meant to change, especially increase, the distance between the insulation panel and the wall but solely to safeguard that the metal sheet is pulled towards the washer of the head for preventing compression of the soft insulation foam around the screw. As a conclusion, this screw of U.S. Pat. No. 3,682,507 is valuable for soft foam panels but does not appear applicable for mounting overlapping panels of fiber cement.
The above example illustrates that screws with specific features that are developed for one use, are not useful for other uses, despite only minor differences. None of the above screws are properly applicable for fiber cement planks/boards and their mounting in imbricated configuration.